1. Field of the Invention
The invention is primarily concerned with method and apparatus for making magnetic recording media using gravure coating to apply magnetizable layers.
2. Description of the Related Art
The magnetizable layers of most magnetic recording media consist of coatings of fine magnetizable particles in organic binder. A magnetizable coating should be of uniform thickness, typically less than 1.0 mil (25 .mu.m), and should be free from defects such as pinholes, streaks, and particle agglomerates. The coating can be applied by a direct gravure coater as illustrated in U.S. Pat. No. 3,761,311 (Perrington et al.), FIG. 1 of which shows "a tank 10 which is continuously supplied with a dispersion 11 of magnetizable particles and binder. This is picked up in the fine grooves of a gravure roll 12 which is scraped by a doctor blade 13 so that substantially the only material left is that contained in the grooves. The dispersion is pressed by a rubber roll 14 into contact with and transferred to an uncoated backing member 15 which is moving at the same speed and in the same direction as the gravure roll 12, as indicated by the arrow 16. Before significant evaporation of the volatile vehicle, the knurl pattern of the coating is smoothed out by a flexible blade 17. The coated backing member then passes between a pair of bar magnets 18 to physically align the magnetizable particles and on to a heated oven 19 to dry the coating" (col. 4, line 65 through col. 5, line 4).
In the Perrington patent, the gravure roll drives both the backing member (below called the "backing web") and a backup roll (called "rubber roll 14" in the Perrington patent), and the magnetizable dispersion is transferred to the backing web in the mirror image of the cellular pattern of the gravure roll. In the quotation from the Perrington patent, that pattern is called "the knurl pattern" because the cells of gravure rolls are often formed by knurling, although other techniques are known such as etching.
Although it does not mention magnetic recording media, Pulkrabek et al.: "Knurl Roll Design for Stable Rotogravure Coating", Chemical Engineering Science, Vol. 38, No. 8 (1983), pages 1309-1314, contains useful information concerning the design of a gravure roll that is to be used in direct gravure coating to deposit a high-viscosity fluid such as a pigmented binder. One of the present inventors is a co-author of the Pulkrabek publication.
The Pulkrabek publication reports tests on a large number of gravure rolls, the cells of which are grooves with helix angles from 30.degree. to 90.degree., tooth angles from 53.degree. to 117.5.degree., and pitches from 5 to 39 grooves/cm. In spite of such variations, the volume of transferred fluid (the pickout) consistently amounted to about 59% of the volume of the grooves. To obtain uniform coatings that are substantially free from defects such as pinholes and streaks, proper or stable pickout is required, and to obtain stable pickout, the fluid in each groove of the gravure roll should transfer as one ridge of fluid as shown in FIG. 3. This is achieved when the "knurl roll coating line frequency" or "imposed ribbing frequency" matches the stable "natural ribbing frequency" of the fluid for the same wet coating thickness. To obtain the stable "natural ribbing frequency" of a fluid, it is deposited on a moving web by a roll coater or a spreader at a gap height that passes a volume of fluid per unit area equal to the volume passed by a gravure roll. As the web speed is increased, the number of ribs per unit width that form naturally per unit width increases until a maximum value is reached asymptotically. That maximum number is the stable natural ribbing frequency of the fluid.
The Pulkrabek publication explains that the "imposed ribbing frequency" is obtained by multiplying the groove pitch of the gravure roll times the sine of the helix angle. The ratio of the imposed ribbing frequency to the natural ribbing frequency is here called the "Ribbing Ratio" or "RR". The Pulkrabek publication says that proper or stable pickout requires an RR close to one. This and other parameters of the gravure roll must be closely controlled to obtain uniform coatings of high-viscosity fluids when the web is being driven by the gravure roll.
Stable pickout can be either "merged" (i.e., adjacent ridges of the coating are interconnected by the applied dispersion) or "open" (i.e., adjacent ridges of the coating are spaced). Whether merged or open, ridges of dispersions of magnetizable particles typically are too high in viscosity to self-level and hence must be smoothed before being allowed to dry. Although open pickout can be smoothed, the smoothing step is enhanced when the pickout is merged.
Although magnetizable layers of excellent quality can be applied while the backing web is being driven by the gravure roll at speeds up to about 400 ft/min (122 m/min), the following problems have been encountered at higher coating speeds: 1) difficulty in filling and doctoring the grooves, 2) misting, 3) the inability to achieve stable merged pickout, and 4) a tendency for very thin backing webs to wrinkle in the longitudinal direction, resulting in nonuniform coating thicknesses. Additionally, at a backing web speed of about 400 ft/min (122 m/min), it has not been feasible to attempt to coat to dry thicknesses less than 0.08 mil (2 .mu.m).
Although each of the above-discussed references concern gravure coating in which the backing web is being driven by the gravure roll, reverse gravure coating has been known at least since the publication of Witt: "Reverse Gravure . . . Part I", Paper, Film & Foil Converter, Vol. 51, August 1977, pages 41-43. The Witt publication does not mention magnetic recording media, but concerns the coating of aqueous dispersions of PVDC [poly(vinylidene chloride)]. Witt says that previously such dispersions had been mainly applied by air knife coating and that misting had been encountered at speeds of 110-130 m/min, a problem not encountered in reverse gravure coating. Witt says that to obtain coatings of good quality, the gravure (applicator) roll should turn at least as quickly as the web (p. 43, left col.). See also Witt: "Reverse Gravure . . . Part II" Paper, Film & Foil Converter, Vol 51, Sept 1977, pages 51-53.
Reverse gravure coating is also discussed in Benkreira et al.: "Gravure Roll Coating of Low Viscosity Liquids", Surface Coatings International JOCCA, Vol. 75, No. 7, July 1992, pages 261-268. The Benkreira publications reports a first set of experiments with the speed ratio of unity (which is--1.0 in reverse gravure coating) and a second set of experiments with various speed ratios by altering the substrate speed while keeping the gravure roll speed at 0.83 m/s (p. 264, left col.).
The following two publications concern reverse gravure coating: Patel et al.: "Gravure Roll Coating of Newtonian Liquids", Chemical Engineering Science, Vol. 46, No. 3, 1991, pages 751-756, and Benkreira et al: "Direct Gravure Roll Coating", Chemical Engineering Science, Vol. 48, No. 12, 1993, pages 2329-2335. Although the first paragraph of each of these two publications mentions the coating thicknesses of "audio, video and computer tapes," nothing further is said about the coatings of such tapes. Instead, each publication states that it considers only Newtonian fluids, whereas coatable dispersions of magnetizable particles and binder are non-Newtonian.